1. Technical Field
Embodiments of the invention relate to systems and methods for providing a multi-phase voltage-controlled oscillator (xe2x80x9cVCOxe2x80x9d).
2. Description of the Background
As the bandwidth of digital data communication increases, the performance of a given communication device""s clock signal generator (i.e., frequency synthesizer) becomes more important. Stated differently, highly stable clock signals, generated by a monolithic integrated circuit, facilitate higher-quality, high-bandwidth communication. For example, radio frequency (xe2x80x9cRFxe2x80x9d) transceivers and data samplers in high-speed serial link receivers are applications that rely on stable clock signals.
One aspect of the stabilizer of a frequency synthesizer is the exactness of the time interval, or of the phase difference, between the various clock signals produced by the synthesizer. Thus, measuring a frequency synthesizer""s jitter in the time domain, or phase noise in the frequency domain, is one indication of the stability of a frequency synthesizer. In other words, low jitter performance, or equivalently low phase noise performance, is an important basis for selecting a frequency synthesizer. A typical frequency synthesizer includes a VCO and other phase correcting feedback circuitry. The jitter performance of the VCO is reflected in the performance of the frequency synthesizer. Various sources of interference give rise to disturbance of the phases of the signals produced by a VCO. The sources of interference can include varying output voltage swings and varying power supply parameters.
A VCO can include cascaded VCO cells. Each VCO cell can include a transconductor, a capacitor and a load. Transconductance is symbolized by gm and is often measured in microsiemens (10xe2x88x926 siemens). By way of example, a transconductance amplifier is an amplifier that supplies an output current proportional to its input voltage. The transconductance amplifier appears to be a current source with a high output impedance which drives a relatively much lower load impedance. Controlling the transconductance, or the load resistance, provides control over the frequency generated by the VCO cell. One of the primary design issues for a VCO is the purity of oscillation produced by the VCO.
B. Kim, D. Helman, and P. Gray describe a clock recovery circuit in xe2x80x9cA 30-MHz Hybrid Analog/Digital Clock Recovery Circuit in 2-um CMOS,xe2x80x9d IEEE J. of Solid State Circuit, vol. SC-25, pp. 1385-1394, December, 1990 (xe2x80x9cKim et al.xe2x80x9d), hereby incorporated by reference. Kim et al. uses source-coupled nMOS transistors to implement a transconductor that converts input voltage into output current. In addition, Kim et al. has pMOS transistors operating as a resistive load. Adjusting the bias of the transconductor controls the transconductance, which in turn varies the oscillation frequency.
However, the voltage swing of a VCO has a strong influence on the generated frequency. More specifically, changes in the amplitude of the voltage of a VCO""s outputs create undesirable jitter. Furthermore, some interface circuits that convert VCO signals into appropriate signal forms operate optimally only if the VCO fixes the amplitude of the voltage swings produced by the VCO. In an attempt to design a oscillator that will produce constant amplitude output voltage swings, one can use a replica bias circuit that mimics VCO characteristics that generates an appropriate bias to maintain relatively constant amplitude output voltage swings. However, this approach has some drawbacks with respect to the noise characteristics of the replica bias circuit. More specifically, the replica""s amplitude control circuit generates substantial noise. This circuit noise eventually causes phase noise and jitter in the waveforms produced by the circuit.
Thus, there is a need for systems and methods for producing stable clock signals with reduced jitter.